Probably when you bumped into the trash can your sensors might have broken The sensor works like a radar...it sends out acoustic signals and determines the distance based on the time lag of sending and receiving the signal.
So if there is a foreign object stuck near the transducer it might prompt a false alarm. I think you should clean the sensor probe first with a mild cleaner as suggested by Winterpeg and also air brush the sensor.
If it still doesn't work then check the wire taps.
Hope this helps.
Jake
Toyota Los Angeles